Low frequency amplifier



6, 143. .J. VAN SLOOTEN ETAL 3,

Low FREQUENCY AMPLIFIER Filed Aug. .7, 1957 2 Sheets-Sheet 11 ros/alvAzsol/R05 70 mm; M SUI/RCE vvvvvvv INVENTQRS JACOB VANSLOOZ'EN up VOLZ.

BY AND TGTI'RNEY y 1943' J. VAN SLOOTEN ETAL 2,323,534

I LOW FREQUENCY AMPLIFIER Filed Aug. 7, 1937 2 Sheets-Sheet 2 SOURCEINVENTORS JACOB m/v .SLOOTEN ND 2 E VOLZ W ATT RNEY Patented July 6,1943 LOW FREQUENCY ALIPLIFIER Jacob van Slooten and Rudolf FriedrichVolz,

Eindhoven, Netherlands, assignors, by mesne assignments, to RadioCorporation of- America, New ,York, N. Y., a corporation of DelawareApplication August 7, 1937,Serial No. 157,927 In Germany August 11, 19363 Claims.

The present invention relates to low frequency amplification systems.

It has for its object to ensure a greater amplification of the strongthan of the weak signals,

owing t. which the reproduction comes nearer to reality. .Such systemsare known; thus, for

example, it has previously been proposed to connect a resistance havinga ,high positive temperature coefiicient in parallel with (the outputcircuit.

Many known systems, especially that above referred to, causedifficulties if, in addition, use is made of negative retroaction,because this re duces'again the variations in intensity. The in-.vention consists in rendering the retroaction dependent on the signalamplitude, namely in such manner that with a small amplitude of thesignal to be amplified the retroaction is greater than with a largeamplitude. This may be achieved by feeding back through an auxiliarytube whose amplification depends on the signal amplitude. A very simplemeans of rendering the retroaction dependent on the signal amplitudeconsists in leading part of the output current through an elementconnected in th retroactive circuit, and having a resistance whichdepends on the current intensity. As such an element use shouldpreferably be made of a resistance having a high temperaturecoeificient,

The invention will be explained more fully with reference to theaccompanying drawings which represent, by way of example, fourembodiments of the invention, in Figs. 1, 2, 3 and 4, respectively.

Figure 1 represents a system comprising two amplifying tubes I and 2which are coupled by means of resistances and a condenser, The outputcircuit of tube 2 includes the primary winding of a transformer 5, tothe secondary winding of which is connected the load, e. g. aloudspeaker 4. According to the invention, retroaction is effectedthrough a tube 3 whose amplification depends, as may appear from thefollowing, on the signal amplitude.

The tube 3 comprises an auxiliary anode I0 which is connected by acondenser 9 to the anode of tub I. The rectified current passes througha plurality of resistances amongst which there is a resistance II acrosswhich an increasing drop of voltage is produced when the signalamplitude increases. One of the ends of the resistanc II is connected tothe grid of the tube 3. Upon simple consideration, it will appear thatwith an increasing signal amplitude the voltage of the grid of the tube3 becomes more negative due to the potential drop across the resistanceII, with the result that the amplification by this tube decreases. Thevoltage across the grid and .the cathode of the tube 3 is operative atthe load 4. The anode of the tube 3 is connected through a resistance I2to the positive terminal of the source of supply, and, in addition, by acondenser I3 to the cathode of the tube I. The anode current of thistube I flows through resistances 6 and I which are connected into thesupply lead of the cathode, and which form at th same time part of theinput circuit of the tube I. I

The resistance I, which has a condenser 8 connected in parallel with it,serves to apply a negative bias to the grid of the tube I. Theresistance 6 acts as a coupling resistance between the anode circuit ofthe tube 3 and the input circuit of the tube I. As will be clear fromthe above, part of the signals operative in the secondary transformerwinding is amplified by the tube 3 and supplied to the input circuit ofthe tube I, a negative retroaction being thus obtained. This negativeretroaction is known per se and may be employed for the suppression ofnon-linear distortions in the amplifier. A condenser I8, as well asresistances I9 and 20, prevent the low-frequen'cy oscillations suppliedto the anode ID from reaching the grid of the tube 3. In addition, theyprevent the oscillations of the output circuit from attaining the anodeI0.

Since the amplification by the. tube 3 depends on the signal amplitudethe retroaction also depends thereon, namely in such manner that with anincreasing signal amplitude the negative retroaction decreases. Thenegative retroaction results in a decrease of the amplification. Withlarge signal amplitude this decrease is consequently less than withsmall signal amplitudes. The consequence thereof is that with largesignal amplitudes the amplification is greater than with smallamplitudes, with the result that the contrast between strong and weakpassages is increased and the reproduction becomes more faithful.

With the system shown in Fig. 2 use is made, instead of an amplifyingtube, of a resistance having a positive temperature coefficient, for'example an incandescent lamp Id. In the system shown, this lamp islocatedbetween one of the ends of the secondary winding of the outputtransformer 5 and the cathode of the tube I. If

required, there may be included, in addition, a resistance I5 o-r/and aresistance 2| which serve to control the effect. These resistances are,however, not absolutely necessary. Furthermore,

there is provided a connection between the other with the result that thcontrast between these passages is increased.

In the case of a low output energy it is generally desirable to make thelow frequencies more prominent than the high frequencies. According tothe invention, this may be effected by connecting into the retroactivecircuit one or more impedances which depend on the frequency. Such acircuit arrangement is shown in Figure 3. It substantially correspondsto that shown in Figure 2 except, however, that a condenser I6 isconnected in series with the incandescent lamp, and that in parallelwith this series-connection is located an impedance I! which may becapacita-' tive or inductive according to the effect desired. With thesystem represented, the negative retroaction is in the case of weakpassages comparatively greater for the high than for the lowfrequencies. As the impedance I! there may be employed, for example, aninductance owing to which it is ensured that the range of the highfrequencies is made somewhat more prominent. The capacity of thecondenser I6 is so chosen that in the case of weak passages, that is tosay when the filament has a comparatively low temperature, the impedanceis for low frequencies large with respect to the resistance of thefilament; whereas in the case of strong passages the resistance in theretroactive circuit is substantially determined by that of the filament.The condenser I6 should have in general a high value; an electrolyticcondenser should preferably be utilized therefor.

Instead of a resistance having a positive temperature coefiicient theremay also be employed a resistance with a negative temperaturecoefllcient. This resistance may be connected in series with theresistance 1. Such a circuit arrangement, with which, in addition, theretroaction is dependent on the frequency, is represented in Fig. 4. Inthis figure the resistance with a negative temperature -coeflicient isdenoted by 22, and impedances by 23 and 24. Relatively to an amplifierwherein the resistance with positive temperature coefficient isconnected in parallel with the loudspeaker, the amplifier according tothe invention affords the advantage that the energy consumed for thecontrol of the sound'intensity is greatly reduced.

Suppose impedance 23 is absent or very high, and 24 is shorted or verylow. Also, let I1 be a resistance. Then the greater the speaker voltagethe more current fiows through 22, and hence the less its resistance.Since the speaker voltage is divided between the drop through I! and thedrop throughi22 a smaller across 22 as the resistance of 22 decreases.As the drop across 22 is the degenerative feed back voltage this meansless degeneration is secured as output increases, or in other wordsrange expension is had. If, now, I 7 is made inductive, as in Fig. 3, it

will by virtue of its impedance increasing with frequency automaticallyreduce the degenerative feed back as frequency increases so that at veryhigh frequencies there is never much degeneration at any amplitude, and,hence, relatively little volume expansion as compared to the expansionfor low frequencies. On the other hand, if I! is a capacity the effectsare the opposite, and there is relatively little expansion at lowfrequencies so that at low intensities the low tones are relatively moreprominent than the higher frequency tones.

proportion will appear generative feed back variations of current Thedependence of the amount of expansion on frequency may be adjusted tofollow more nearly to any desired law by providing further frequencydiscriminatory elements such as 23 and 24. For example, if H and 24 arecapacities, while 23 is an inductance, a high pass filter structure isformed whose input receives voltage from the speaker and whose output isconnected to 22. With this arrangement a whole range of low frequenciesmay be substantially freed from deresultlng from amplitude variationwhile a wide range of higher frequencies are fully expanded with respectto amplitude. Contrariwise the use of resistors at 23 and 24 reduces theamount of variation with frequency that a condenser at H tends tointroduce into the expansion by reducing the amount through 22, and alsoby reducing the variation of the total resistance in series with I!resulting from changes in volume.

What is claimed is:

1. In an audio amplifier system of the type comprising at least twocascaded amplifier tubes, a volume range expander network comprising anegative audio feedback path connected between the output circuit of thesecond tube and the input circuit of the first tube, means operativelyassociated with said path for decreasing the feedback of audio voltageto said input circuit in response to increase of the audio amplitude,and additional means operatively associated with said feedback path forincreasing the feedback of audio voltage of high audio frequencies to agreater extent than voltage of low audio frequencies in response to adecrease of the said amplitude.

2. In an audio amplifier system of the type comprising at least twocascaded amplifier tubes, a volume range expander network comprising anegative audio feedback path connected between the output; circuit ofthe second tube and the input circuit of the first tube, meansoperatively associated with said path for decreasing the feedback ofaudio voltage to said input circuit in response to increase Of the audioamplitude, and additional means operatively associated with saidfeedback path for increasing the feedback of audio voltage of high audiofrequencies to a greater extent than voltage of low audio frequencies inresponse to a decrease of the said amplitude, said first meanscomprising a resistance device hav-- ing a positive temperaturecoefficient, and said additional means including a reactive impedanceconnected across the resistance device.

3. In an audio amplifier system of the ,type comprising at least twocascaded amplifier tubes, 9. volume range expander network comprising anegative audio feedback path connected between the output circuit of thesecond tube and the input circuit of the first tube, means operativelyassociated with said path for decreasing the feedback of audio voltageto said input circuit in response to increase of the audio amplitude,and additional means operatively associated with said feedback path forincreasing the feedback of audio voltage'of high audio frequencies to agreater extent than voltage of low audio frequencies in response to adecrease of the said amplitude, said first means comprising the filamentof an incandescent lamp, said additional means including a reactiveimpedance in shunt with the lamp, and a condenser in series with thelatter.

JACOB VAN SLOOTEN. RU'DOLF FRIEDRICH VOLZ.

